For 1 Light in the Darkness, Thorim is the keeper selected to provide assistance for the fight since he is able to kill the adds in phase 3. However the absence of the other 3 keepers means that this encounter becomes significantly harder, especially phase 2, where the absence of Mimiron's powers means that the Corruptor Tentacles can place debuffs 3 times as fast. Of course they are still dealt with using the same strategy, and the priority for tentacles in phase 2 remains unchanged.
Phase 3 is slightly different however, as the absence of Freya's powers means that sanity can no longer be restored. This means that facing the boss during Lunatic Gaze is no longer an option. Therefore most if not all of the adds that appear in phase 3 must be killed while facing away from the boss. For me I was lucky to have started phase 3 on 30 Sanity so that I can afford to lose a few more Sanity while learning to adjust, since this was actually my first time reaching phase 3. By the time I became adjusted to everything I only had 6 Sanity left.